Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1999-07-01
2001-05-22
Smith, Jeffrey A. (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S020140, C623S018110
Reexamination Certificate
active
06235060
ABSTRACT:
The invention presented relates to an artificial joint, in particular an endoprosthesis for replacing natural joints, comprising at least two artificial joint parts, each with curved articulation faces on which the joint parts articulate with respect to one another.
Such an artificial joint is known from the German patent application P 42 02 717.9. In it, the joint faces have different circular section contours in planes which are perpendicular to each other, namely one longitudinal plane and a transverse plane, where the ratios of curvature for the articulation faces in each of the planes are convex—convex, convex-concave or concave—concave, and the geometry of the joint's articulation faces with respect to one another is determined in each of the two planes by a joint chain with two joint axes, a so-called dimeric joint chain, which runs through the centers of curvature of the articulation faces of their respective section contours. Since the articulation faces of this artificial joint are formed convex-concave, concave—concave or convex—convex, essentially point-like load transmission regions come into being so that increased surface pressures can occur on the articulation faces which lead to wearing away of material. This can shorten the life span of these artificial joints. In order to improve the load transmission between the articulation faces of the parts of the joint, it is suggested in the known joint to arrange a pressure distribution body between each of the individual parts of the joint with which a better and more even distribution of pressure is achieved. However, this pressure distribution body increases the number of required joint parts for the artificial joint.
The object of the invention presented is to create an artificial joint in which point-like load transmission regions are avoided and which it is not necessary to fit with pressure distribution bodies and which at the same time conforms optimally to the conditions of the human body when used as an endoprosthesis, particularly substituting a natural human joint.
According to the invention, this is achieved with an artificial joint of the kind described in the introduction, where on each of the articulation faces a curved contact line is formed, and the curved contact line of one of the articulation faces is part of an elliptical section contour of a first cylinder or a cone having the cylinder radius R
1
or respectively the cone angle &agr;
1
, and the other contact line takes the form of a counter track of a second cylinder or second cone having the cylinder radius R
2
or respectively the cone angle &agr;
2
and rolling and/or sliding on the first cylinder or first cone; also the articulation faces comprise control faces formed from a plurality of straight contact lines, and, on one side, these control faces adjoin the contact lines opposite one another, and each of the contact lines is the connection line between an instantaneous contact point of the contact lines and an instantaneous reference point of the respective movement systems in a reference plane or on a reference sphere in the moved/unmoved system. In accordance with the invention, a fixed or moving point of the moved or unmoved system will preferably be chosen as reference point, where it is assumed that one of the cylinders or cones is stationary and the other cylinder or cone rolls and/or slides on this stationary cylinder/cone.
The restriction to an unavoidable motion and the choice of the instantaneous axis of rotation as reference point cause, since the holoids roll on each other without sliding, this characteristic to be transmitted to corresponding sections of the control faces. If, instead of the instantaneous center of revolution, another instantaneous common point is used from the reference plane or reference sphere in the moved or unmoved system, the ratio of rolling to gliding can thereby be varied.
According to the invention, it is further provided that the first and the second cylinder or the first and the second cone are arranged with respect to each other so that they form a straight dimeric joint chain or an overlaid dimeric joint chain. For cylinders in a straight dimeric chain, the relationship R=R
2
+R
1
applies or, in an overlaid dimeric chain, the relationship R=R
2
−R
1
, where R is the radius of the joint's axial path and R
1
the radius of the first cylinder and R
2
the radius of the second cylinder. In the case of the cone, analogous to the cylinders, &agr;=&agr;
2
+&agr;
1
and &agr;=&agr;
2
−&agr;
1
apply, where &agr; is each of the angles between the axes of the tangential cone pairs.
Since, according to the invention, the cylinders or cones roll or slide on or in each other and the distance between the cylinder axes or the angle between the cone axes remains constant, a flat or spherical dimeric chain results. Thus, the motion, in principle having three parameters and possible as flat or spherical, is limited to two degrees of freedom. The corresponding radii of the cylinder or angles of the cone are preferably adapted to the anatomical conditions of the human knee joint, however can also be changed according to the materials used and their characteristics.
The artificial joint according to the invention distinguishes itself in that a linear load transmission region is formed in every contact position of the articulation faces.
Furthermore, it can be advantageous according to the invention if next to the region of direct load transmission a region without any touching contact is formed in the region of the control faces, such that in action the surrounding tissue is only minimally injured. Therefore, according to the invention it is advantageous if, on the side of the contact lines opposite the control faces, the contact lines are extended arched in bowed lines so as to form toric surfaces. The curved lines are defined in that at the instantaneous contact point of the contact lines in the moved or unmoved system a plane is formed which stretches from the respective contact line of the control faces and the common perpendiculars of the contact lines to a tangent of one of the contact lines at the respective contact point. The toric surfaces are formed such that the outer part of the toric surfaces' articulation faces don't touch each other. These outer parts of the toric surfaces form the curved articulation faces' region of indirect load transmission.
According to the invention, the curved lines are attached to their respective contact lines of the control face without any kink, and their normal lines coincide at the contact point. By advantageously forming the curved lines in circularly arcuate sections—if differently sized radii are chosen in the outer sections—a free space can always be maintained between the articulating toric surfaces during the moving of the articulation faces.
Preferably, the joint according to the invention is used in a four-joint as an endoprosthesis for the human knee such that the medial joint compartment forms the overlaid dimeric chain, and the lateral joint compartment the straight dimeric chain, whereby a four-joint is inevitably formed.
Using the exemplary embodiments shown in the enclosed drawings, the invention will be explained in detail.
REFERENCES:
patent: 4888021 (1989-12-01), Forte et al.
patent: 5133758 (1992-07-01), Hollister
patent: 5203807 (1993-04-01), Evens et al.
patent: 5336267 (1994-08-01), Kubein-Meesenburg et al.
patent: 5507820 (1996-04-01), Pappas
patent: 5556432 (1996-09-01), Kubein-Meesenburg et al.
patent: 5738686 (1998-04-01), Kubein-Meesenburg et al.
patent: 5800370 (1998-09-01), Kubein-Meesenburg et al.
patent: 5879390 (1999-03-01), Kubein-Meesenburg et al.
patent: 4310968 (1994-10-01), None
patent: 522822 (1993-01-01), None
patent: 94022396 (1994-10-01), None
Kubein-Meesenburg Dietmar
Naegerl Hans
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
HJS Gelenk System GmbH
Smith Jeffrey A.
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